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Electronic Structure and Physical Properties of 13C Carbon Composite

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Electronic Structure and Physical Properties of 13C Carbon Composite Electronic structure and physical properties of 13C carbon composite ABSTRACT: This review is devoted to the application of graphite and graphite composites in science and technology. Structure and electrical properties, as so technological aspects of producing of high-strength artificial graphite and dynamics of its destruction are considered. These type of graphite are traditionally used in the nuclear industry. Author was focused on the properties of graphite composites based on carbon isotope 13C. Generally, the review relies on the original results and concentrates on actual problems of application and testing of graphite materials in modern nuclear physics and science and its technology applications. Translated by author from chapters 5 of the Russian monograph by Zhmurikov E.I., Bubnenkov I.A., Pokrovsky A.S. et al. Graphite in Science and Nuclear Technique// eprint arXiv:1307.1869, 07/2013 (BC 2013arXiv1307.1869Z Author: Evgenij I. Zhmurikov Address: 68600 Pietarsaari, Finland E-mail: [email protected] KEYWORDS: Structure and properties of carbon; Isotope 13C; Radioactive ion beams 2 1. Introduction In order to explore ever-more exotic regions of the nuclear chart, towards the limits of stability of nuclei, European nuclear physicists have built several large-scale facilities in various countries of the European Union. Today they are collaborating in planning of a new radioactive ion beam (RIB) facility which will permit them to investigate hitherto unreachable parts of the nuclear chart. This European ISOL (isotope-separation-on-line) facility is called EURISOL [1]. At the present moment experiments with RIB of the first generation have yielded important results. But the first generation RIB facilities are often limited by the low intensity of the beams. This is why there have been discussed and developed quite a number of new projects with a new generation RIB (KEK, Tsukuba; ARENAS REX ISOLDE, CERN; ARGONNE; HRIBF, Oak Ridge). The SPES project at LNL (Italy) also belongs here [2, 3]. The proposed project is aimed at an R&D study of a conversion element in the framework of the SPES program at LNL. LNL has proposed a double-acceleration scheme of producing an ISOL type RIB. A primary proton beam accelerated at a superconducing RFQ linac is directed to a special neutron target and produces an intense (3×1014 cm-2×s-1) flux of fast neutrons. The parameters of the primary beam are: the energy is up 100 MeV, the average power is up to 300 kW, the diameter is 1 cm. Thus obtained neutron flux then comes to a hot thick target of a 238U compound. The vapors of the radionuclides get ionized, extracted from the target at an energy of 20-60 keV and then being separated by isotopes get into the experimental zone for low-energy experiments or further accelerated to an energy of up to 1-5 MeV/nucleon. The conversion element consisting of a neutron target and a source of radioactive ions plays a the most important role in this scheme [4-7]. Version converter with a target made of a carbon composite with a high content of the isotope 13C was developed for the proton primary beam. The threshold of the 13C nuclear reaction 13C (p, n) 14N with yield neutrons is of the 3.24 MeV, while reaction threshold 12C (p, n) 13N reaction for usual carbon is significantly above and equal to 20.1 MeV [8, p .895]. It is supposed, the neutron yield from a carbon target that made of pure isotope 13C, can be considerably higher if energy of the proton beam is smaller than 20 MeV. However, it is turned out quite quickly that at higher energies this difference is not so noticeable [9]. Nevertheless, in principle, such a cooled by radiation target can provide in 3-10 times more high neutron yield than natural carbon 12C target at low energies of the proton beam. An argument in favor of such a target is that the using of the deutron beam, as well as an increase of its power, it will be resulting into a significant increase of the cost and complexity of the project EURISOL in whole. 3 Moreover, the 13C isotope composite finds out its application in the resonance gamma spectroscopy [10, 11]. This method is based on that the nuclei of many chemical elements have the property of resonance absorption. In particular, the nucleus of the nitrogen atoms can resonantly absorb gamma quanta with energy of 9.1724 MeV, and the width of the absorption peak is of 125eV only. Method of nitrogen detection is found in comparison to the resonance and non-resonant absorption when a radiation comes through the matter. Nuclear reaction 13C (p, ɤ) 14N is used to generate a resonance ɤ-quanta. In this case accelerated up to 1.75 MeV proton beam bombards the graphite target with a high content of 13C carbon isotope, therefore it is formed an excited nucleus 14N that emits gamma quanta with energy of 9.17 MeV. In this case, the necessary energy for resonance spectrometry has gamma-quanta with emission angle 80,7 ±0,10 to the axis of the proton beam [11]. This review summarizes the results of studies of an electronic structure and properties of 13C carbon composite, because it can allow to predict its lifetime and its possibilities as a construction material. Samples of this composite with a high content of the 13C isotope were made in "NIIgraphit"[12] of powder that was obtained by chemical vapor deposition. Properties of the graphitization and a technological scheme of the graphite preparation were described earlier [13, 14]. 2. X-ray and high-resolution microscopy of the 13C carbon composite X-Ray measurements and high-resolution microscopy measurements were carried out and described by leading of prof. S.V. Tsybulya in the Boreskov Institute of Catalysis SB RAS and published earlier in [15]. It was used URD-6 with monochromatic CuKα- radiation. The registration of diffractograms was performed in steps mode with step 0.05o, the accumulation time is 10 seconds and the angle range 2θ was from 10 to 100o. The XRD profile of the 13C carbon composite is shown on fig. 1. It is clear visible 00l and hk0 reflection (line 2) of graphite planes, the hk0 reflexes have an asymmetrical shape with a large blurring towards larger angles than in the case of high-ordered polycrystalline graphite (line 1). This diffraction pattern complies to the turbostratic graphite structure in which there are not ordered graphene layers along the c - crystallographic direction [16]. The diffractogram of 13C carbon isotope really shows (line 3) really only one well-defined broad diffraction peak, that complies to the 002 reflex of the graphite structure. Enlarged image of the diffraction pattern allows us to identify a 4 broad peak 100, located at 2θ =44° (fig. 2). The size of the coherent scattering region CSR (Å) for the structural components of the 13C powder isotope is shown in table 1. Table 1. The dimensions of the coherent scattering region (Å) for the structural components of the initial 13C powder A more detailed structure of the 002 reflex of the 13C powder is shown in the inset to fig. 2. The presence of an inflection indicates that the powder is composed of two structural components, which differ in size. Indeed, the 002 peak is a superposition of two components, relating to the graphite particles with CSR of 20Å and 40Å and different interplanar distance d002 (table 1). Similar values of integral component of intensities indicate to equal proportion of these particles in the sample. Low intensity of 100 peak does not allow to attribute it to one or another of the structural component. Fig. 1. The X-Ray phase diagram: 1) MPG-6; 2) the initial tablet of 13C carbon composite 3) initial powder of 13C pure isotope [17]. 5 Fig. 2 The X-Ray phase diagram of the initial powder of 13C pure isotope. The shape of the diffraction 002 peak (inset at top) indicates the presence in the sample of two structural components with different dispersion and, probably, with different interplanar spacing d002. [17]. High-resolution transmission electron microscopy (HRTEM) is shown that the sample of 13C carbon composite of density less than 0.8 g/cm3 consists of particle aggregates with size up to 1000 nm (fig. 3a). However, the morphology of particles that make up the aggregate is really different from carbon composite of MPG class, because each thin plate is arranged so, that looks like a sheet of paper, that was crumpled in the middle, and then a little straightened [15]. Structure of the plate is not a monocrystalline, and complies a polycrystalline state: a set of randomly oriented interconnected blocks, giving the ring microdiffraction that is shown on the inset to fig. 3 a. It is interesting that curving edges of such plates are similar to carbon fiber that can be clearly seen in the micrograph of larger scale (fig. 3,b). Constructional composites are based on 13C isotope with high density (ρ ~ 1.55 g/cm3), and isotope content of from 50% up to 75% are composed of three types of carbon particles. The main mass of the samples are the crumpled and broken graphite plate with thickness from of 1 to 50 nm, which have a tendency to agglomerate. Moreover, the sample contains graphite globules, which are generally well faceted and have the sizes from 50 to 150 nm; some globules are partially destroyed. Every facet of the globule is a graphite plate of thickness about 15-20 nm. 6 Fig. 3. The micrographs and microdiffraction of 13C carbon composite of density less than 0.8 g/cm3 [15].
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